1. Field of the Invention
The present invention relates to a coil-embedded dust core having a structure in which a metal coil is covered with a soft magnetic alloy powder compact.
2. Description of the Related Art
Requirements for small and high-performance dust cores to be mounted on electronic equipment have become intensified as miniaturization and weight reduction of the electronic equipment have been advanced. The dust core is produced by molding a soft magnetic alloy powder, e.g., a ferrite powder, having a high saturation magnetic flux density into a desired shape through compaction.
In order to produce a smaller and higher-performance inductor provided with this dust core, it has been proposed to construct a structure in which a metal coil is embedded in the inside of a dust core by embedding the metal coil in a soft magnetic alloy powder and compression-molding the entirety in that state.
The inductor having the above-described structure can be referred to as a coil-embedded dust core. In a technology known as an example of a method for producing this type of coil-embedded dust core, as shown in FIG. 10, a pressure device provided with an upper punch 103 and a lower punch 104 in a frame 102 composed of an upper frame 100 and a lower frame 101 is used, a soft magnetic alloy powder is put into a space enclosed by the above-described frame 102, the upper punch 103, and the lower punch 104, followed by being compacted, so that a lower core 106 is molded once. Subsequently, a metal coil 107 is disposed on this lower core 106, the soft magnetic alloy powder is filled in again to embed this coil 107 and, thereafter, as shown in FIG. 11, the entirety is compacted again with the upper punch 103 and the lower punch 104, so that an inductor 110 having a structure in which a metal coil 107 is embedded in the inside of a dust core 109 is produced (refer to Japanese Unexamined Patent Application Publication No. 2001-267160 corresponding to U.S. Patent Application Publication No. U.S. 2001/0016977 A1).
The inductor 110 having the structure in which the coil 107 is embedded in the inside of a dust core 109 integrally including the lower core 106 molded in advance can be produced by the method described in Japanese Unexamined Patent Application Publication No. 2001-267160.
In a technology known as another example of a structure of the above-described coil-embedded dust core and a production method therefor, as shown in FIG. 12, a coil 115 having a structure in which a coil portion 111 is formed by winding edgewise a flat type wire in such a way that a long side is arranged perpendicularly to a winding axis, and terminal portions 112 and 113 are disposed by extension at both end portion sides of the coil 115 is used and, as shown in FIG. 13, terminal portions 112 and 113 of the coil 115 are held between an upper mold 116 and a lower mold 117, so that the coil portion 111 is contained in the inside of the molds 116 and 117. A soft magnetic alloy powder 118 is filled in an inside space of the molds 116 and 117 and, thereafter, the soft magnetic alloy powder 118 is compacted with an upper punch 120 and a lower punch 121 (refer to Japanese Unexamined Patent Application Publication No. 2004-153068).
An inductor 123 having the structure in which the coil portion 111 is covered with a dust core 122 and terminal portions 112 and 113 are protruded to both sides of the dust core 122 can be produced by the method described in Japanese Unexamined Patent Application Publication No. 2004-153068. The inductor 123 is completed by bending and placing the terminal portions 112 and 113 on the bottom surface side of the dust core 122 in consideration of mounting on wiring boards and the like.
Furthermore, a structure composed of a coreless coil 131 disposed by spirally winding a tabular conductor wire 130 made of a flat type conductor wire or a foil-shaped conductor wire in such a way that the right side and the back side are faced each other, a terminal stage 132 on which the coreless coil 131 is mounted, soft magnetic alloy plates 134 and 135 to sandwich them from top and bottom, and an insulating sheet 136, as shown in FIG. 15, is known as an example of a structure of a choke coil of a type different from the coil-embedded dust core having the above-described structure (refer to Japanese Unexamined Patent Application Publication No. 10-125545 corresponding to the U.S. Pat. No. 6,774,755 B2).
When the structure of the known inductor 110 described above with reference to FIG. 10 and FIG. 11 is adopted, two steps of molding operation are required. For example, the lower core 106 is formed in the first molding by using the upper and lower punches 103 and 104 and, thereafter, the entire dust core 109 is molded again in the second molding. Therefore, there are problems in that two steps of molding operation are required, and the production is not easy.
As for the structure of the known inductor 110, the soft magnetic alloy powder is filled in around the coil 107 and are compacted while both ends 107a and 107b of the coil 107 are lead to the outside the coil 107 and are held between the upper frame 100 and the lower frame 101. Therefore, the positions of the upper and lower punches 103 and 104 must be precisely controlled in such a way that both ends of the coil 107 are not torn during compaction of the soft magnetic alloy powder with the upper and lower punches 103 and 104, the mold itself must be divided into components of the upper and lower frames 100 and 101, the configurations of the frames become complicated, the facilities become expensive, the production becomes complicated, and there is a problem in that the cost is not readily reduced. A problem similar to this problem occurs in the structure and the production method described above with reference to FIG. 12 to FIG. 14, and there is a problem in that it is difficult to produce through only one time of compaction.
As for the structure shown in FIG. 14 provided with left and right terminal portions 112 and 113, no problem occurs when the dust core 122 having an adequate vertical thickness in the thickness direction of the terminal portions 112 and 113 is disposed in the structure, as shown in FIG. 14. However, in the case where the electronic product has the dimension of about 5 mm or a few millimeters, that is smaller than 5 mm, in thickness and about 5 mm in width in accordance with the requirement for miniaturization of the electronic equipment and, therefore, the dust core 122 having an adequate vertical thickness in the thickness direction of the terminal portions 112 and 113 cannot be disposed, a load is applied to end portions of the dust core 122 when the left and right terminal portions 112 and 113 are subjected to bending, so that chipping or cracking may occur at the end portions of the dust core.
For example, since a dust core portion located under the base of the terminal portion 113 lead from the bottom side of the coil portion 111 has a particularly reduced thickness, there is a high probability that chipping or cracking may occur at this reduced thickness portion when the terminal portion 113 is subjected to bending. In particular, when the dimension of a portion including the dust core 122 is about 5 mm square in this type of inductor, the thickness of the entire dust core 122 is on the order of a few millimeters. Therefore, the above-described reduced thickness portion may become a particularly weak and brittle portion.
As for the structure of the coreless coil 131 provided with the tabular conductor wire 130 described above with reference to FIG. 15, an end portion of the tabular conductor wire 130 on the inner perimeter side of the coreless coil 131 is lead to the bottom side so as to constitute an inner terminal portion 137, an end portion of the tabular conductor wire 130 on the outer perimeter side of the coreless coil 131 is lead to the bottom side so as to constitute an outer terminal portion 138, and the top and bottom of this coreless coil 131 are sandwiched by the soft magnetic alloy plates 134 and 135. Therefore, there is a problem in that this structure cannot be a simple dust core structure. For example, if the coreless coil 131 having the above-described structure is mounted on a device provided with the upper and lower punches and the upper and lower frames and is pressurized from the top and the bottom, since the tabular conductor wire 130 is disposed in such a way that the width direction is aligned along the direction of pressurizing with the upper and lower punches, when compaction is performed at a high pressure with the upper and lower punches, the tabular conductor wire 130 having the winding structure may be partially buckled. Therefore, there is a problem in that it is essentially difficult to compact while the shape of the coil is precisely maintained.